Titanium or stainless steel has mainly been used as a material for fabricating a wristwatch case. Especially in recent years, the use of titanium and a titanium alloy, which exhibit excellent characteristics in corrosion resistance, strength, lightness and biocompatibility, in exterior parts of a wristwatch is increasing.
For example, the above titanium or stainless steel is useful as a material for fabricating a wristwatch case or a pipe for stem fixing fitted in a wristwatch case. A wristwatch case having a titanium or stainless steel pipe and a method of fixing a pipe to a wristwatch case now present themes which are important from the viewpoint of a long-term use of wristwatch.
Specifically, when it is intended to fix a pipe to a wristwatch case, it is generally needed to provide the step of making a through-hole, as a prepared hole for passing wristwatch case. With respect to this stem hole, for example, it is required that, when a stem furnished with an O-ring is set and fixed, a satisfactory air tightness be realized and maintained at the interface of O-ring and case.
When titanium, a titanium alloy or stainless steel is used as a wristwatch case material, furnishing a wristwatch case of such a material with a through-hole by means of, for example, a drill would often result in roundness deterioration and failure to obtain smooth wrought surface because of the poor machinability of the material.
Accordingly, in order to prevent the deterioration of tightness and decrease of pipe fixing strength attributed to the deterioration of roundness and deterioration of wrought surface smoothness, there is a demand for a method of providing a pipe of higher precision than that of a through-hole made in a case and fixing the pipe to a case by some means.
For example, in order to enhance the dimensional accuracy, it is customary to use a titanium alloy such as Ti-6Al-4V with respect to a titanium case and a free-cutting stainless steel containing a minute amount of sulfur component with respect to a stainless steel case as a pipe material of a free-cutting material of high strength.
Further, when it is intended to fix a separately produced pipe to a through-hole of wristwatch case, it is customary to employ, for example, a method of rendering the outside diameter of the pipe larger than the diameter of case through-hole and press fitting the pipe in the through-hole so as to attain fixing thereof. For example, referring to FIG. 16, through-hole 41a is made at a desired position of wristwatch case 41 constituted of titanium by the use of a drill. This is a hole for inserting therein a stem to be coupled with a main body known as wristwatch module. Conventional through-hole has a diameter of about 2 mm. The through-hole is made at a side of wristwatch case at 3 o'clock position. Thereafter, pipe 42 of a titanium alloy having an outside diameter slightly larger than the diameter of the through-hole is produced and press fitted in the through-hole.
However, even if the above titanium alloy or free-cutting stainless steel is used as a wristwatch case material, drilling of the material for making a through-hole is often accompanied by an extreme degradation of working accuracy attributed to welding or the like, and further accompanied by a decrease of roundness of created through-hole attributed to a not necessarily flat structure of wristwatch case.
In the method wherein a pipe fixing is carried out only by the above press fitting, due to, as aforementioned, the roundnesses of pipe circumference and through-hole made in the case and the surface roughness magnitude thereof, an interface exists between the wristwatch case 41 and the pipe 42 because of the influence of the dispersion of dimensional accuracy thereof depending on a combination of the through-hole 41a of the wristwatch case 41 and the outside diameter of the pipe 42. Furthermore, at the interface depressed by the press fitting, a minute gap remains even after the occurrence of partial plastic deformation. These have caused such problems that a stable pipe fixing strength cannot be obtained and further the waterproof sealing properties are deteriorated.
Therefore, in the related art, the waterproofing capability has been ensured by applying adhesive 43 around the gap and thereafter heating at given temperature so as to dry and fix the same.
However, the adhesive is often constituted of organic materials and is likely to suffer deterioration or the like with the passage of long time. Thus, the pipe fixing strength cannot be stably maintained for a prolonged period of time, and further it has been difficult to ensure the waterproofing capability for a prolonged period of time. Moreover, the sealing by the adhesive exhibits high steam permeability, thereby posing such a problem that the moisture proofing capability is poor.
Among other methods of fixing a pipe to a wristwatch case, a fixing method wherein welding is used has been tried. In this method, a projection is provided on the circumference of the pipe and is selectively fused and joined to the wristwatch case.
However, this method poses such a problem that, because of the need to provide a projection on the pipe, the size of the pipe is increased so as to disenable general employment of the method in wristwatches of ordinary size and, further, high cost is inevitable. Moreover, when the projection provided on the pipe does not come into contact with the wristwatch case uniformly all round the circumference thereof, there have been such occasions where the current for welding cannot flow uniformly causing the welding operation to be difficult.
Still further, among other pipe fixing methods, a method wherein brazing is performed has been tried. In this fixing method, first, a through-hole for inserting a stem therein is made at a desired position of a side of wristwatch case. A pipe is then provided. Next, a silver brazing material (Japanese Industrial Standard (JIS) BAg-8 (melting point 780° C.) or the like) is applied on the internal surface of the through-hole or the outside surface of the pipe, and the pipe is fitted in the through-hole. Thereafter, heating is carried out in an electric furnace or a vacuum oven at temperatures higher than the melting point of the brazing material, so that the brazing material is fused to thereby effect brazing of the through-hole and the pipe. In this method, the fused brazing material flows into minute interstices between the through-hole and the pipe and is cooled to thereby solidify. Thus, fixing of the pipe while ensuring airtightness can be accomplished. That is, the brazing material when heated is liquid and can be satisfactorily penetrated into the interstices between the through-hole and the pipe as long as satisfactory wettability is ensured, thereby enabling realization of high waterproof sealing properties.
However, this method poses such a problem that, when, for example, a silver brazing material is used, the silver brazing material is generally constituted of a material containing Cu in a high proportion with the result that satisfactory results cannot be obtained in a corrosion resistance test, such as an artificial sweat test, for wristwatch.
Moreover, in recent years, a brazing material of Ti—Cu—Ni is used as a brazing material for titanium member wherein the content of Cu is reduced. The melting point of this brazing material is in the vicinity of the transformation point of titanium, so that the brazing material has a drawback in that, with respect to, for example, a titanium wristwatch case having undergone a specular finishing before brazing, the surface condition is changed upon brazing. Specifically, the titanium crystal has a hexagonal closest packing structure at ordinary temperatures but assumes a body-centered cubic structure at 882° C., which is the transformation point, or above. Thus, heating at the transformation point or above causes the titanium crystal to be coarse. Therefore, when an operation performed at temperature exceeding the transformation point is included in the process for producing a wristwatch case, for example, fixing a pipe to a wristwatch case, the surface condition is changed by this crystal transformation.
The wristwatch is also appreciated as an ornament, and, with respect to the surface condition, there are various demands, for example, demands for mirror, hairline, honing and matte finishing (satin finishing). Thus, changing of the surface condition at the time of fixing a pipe is unfavorable in the process for producing a wristwatch. In the event of re-polishing of a wristwatch case, it poses such a problem that the process becomes so complex that the practicality thereof would be lost.
On the other hand, with respect to stainless steel, heating at temperature generally ranging from 450 to 850° C., especially from 600 to 850° C., would cause precipitation of Cr carbide to occur at crystal grain boundary. At portions where the Cr carbide has been formed, the chromium oxide of high corrosion resistance would be in a deficient state. It is known that, as a result, such portions would exhibit a locally extremely deteriorated corrosion resistance, thereby becoming the cause of corrosion. This is known as sensitization phenomenon. Therefore, when it is intended to use stainless steel, heating at temperature which falls within the above range must be avoided.
The present invention has been made with a view toward solving the above technological problems of the related art, and an object of the present invention is to provide a wristwatch case which is excellent in waterproofing capability and corrosion resistance and exhibits a long-term reliability regarding a pipe fixing and to provide a method of fixing a pipe to such a wristwatch case.